This invention relates to a method of positional alignment of substrate and screen mask in screen printing of electrical paste such as cream solder, conductive paste and so on.
On the substrates on which electronic parts are to be mounted, electrical pastes including cream solder for soldering the electronic parts, conductive paste for forming circuit patterns and insulating paste are applied by means of screen printing.
In the screen printing, a screen mask is put on a substrate and then a squeegee is made to slide on the screen mask so as to apply the electrical paste through pattern holes formed in the screen mask onto predetermined portions of the substrate.
When performing the screen printing, it is necessary to make accurate positional alignment of the substrate and screen mask, and therefore the positional alignment of the substrate and screen mask will precede the screen printing. The term of "positional alignment" mentioned in this specification means to obtain relative positional discrepancy between the substrate and the screen mask which is to be used as reference when the screen mask is replaced by a new one or when the screen mask is replaced by that for the next model at the of model change. Heretofore, such positional alignment has been performed in the following manner. Namely, an operator observes peculiar portions of the screen mask with a camera while moving the camera relatively and horizontally with respect to the screen mask so as to perform the teaching about the position of the screen mask. For the substrate as well, its peculiar portions are observed with a camera in the same manner so as to perform the teaching about the position of the substrate. Then, the relative positional discrepancy between the screen mask and substrate is calculated, the result of which is registered in a memory of a computer as position correction data. Thereafter, an actual printed substrate is conveyed so as to undergo the screen printing.
However, according to the conventional method, it has been customary to perform the positional alignment of the substrate and the screen mask while moving the camera horizontally with respect to the screen mask and the substrate by the hand of the operator, so that lots of time and labor were required and errors appeared easily depending on the degree of skill of the operator.